The heat factor is known to be decisive in the acceleration of concrete cure. Otherwise concrete would gain strength too slowly, especially in case of low ambient temperatures.
Various heating methods prevail among the existing methods of concrete heat treatment. These methods feature a long duration of heat treatment, considerable energy consumption and disruption of concrete structure because of thermal expansion of air and water vapour present in the concrete mix.
The evergrowing pace of modern construction calls for new, more efficient heat treatment methods contributing to an acceleration of concrete hardening and improvement of its quality, and the acute energy problem imposes more stringent requirements upon energy consumption, including energy consumption for heat treatment of concrete.
Known in the art is a method for processing concrete mix (cf. USSR Inventor's Certificate No. 1087496, Int.Cl. C 04 B41/30, publ. in Off. Bull. No. 15, 1984). This method comprises continuously moving the mix within a closed vessel, heating the mix during movement within a limited zone along the mixer by supplying steam from the outside. The mix is moved in the steam treatment zone at a velocity of 45-55 cm/s, the steam pressure being 0.7-1.0 kgf/cm.sup.2.
An external heat source -- steam is required for carrying out this method so that the heating control and provision of pre-set process parameters of hot concrete mix are rather complicated. The contact between the steam supply systems and concrete mix results in a rapid deposition of concrete mix on such systems so that efficiency and stability of operation of an apparatus carrying out the method decreases. The mix is caused to move by a screw driven by an electric motor so that total energy consumption for heat treatment of the mix by this method increases. In addition, when mix is heated to high temperatures, the mix sticks to the screw thus hampering the mix heating, movement and stirring.
Known in the art is a method for semicontinuous processing of gypsum (cf. Stevens Norbert J. Semicontinuous Material Treatment Process.Joy Manufacturing Co., U.S. Pat. No. 3158441, Cl. 23-123, filed Mar. 7, 1962, publ. Nov. 24, 1964).
This method is carried out in a calcination apparatus comprising a vertical cylinder vertically divided by rigid partitions into five compartments. The partitions have hollow guide members which also function as heat exchangers.
A vertical shaft extends through the central part of the cylinder to rotate it. Dihydrous gypsum is admitted through a top cone of the cylinder to the first compartment where water available in gypsum evaporates under the action of high temperature (260.degree. C.). The process of evaporation from gypsum is carried out by heating the surface of heat exchangers through which hot oil circulates in any appropriate known manner.
The dried gypsum descends into the next compartment under pressure of steam formed in the compartment. Pressure in the first compartment drops to normal, and a new batch of gypsum gets to this compartment. In the second compartment, wherein temperature is about 325.degree. C., gypsum is decomposed, the chemically bound water is removed, and an equilibrium vapour pressure is established. The calcination process is completed in the fifth (bottom) compartment.
Calcination of gypsum by this method is carried out under pressure of water vapour released from gypsum during calcination. Gypsum is calcined both directly by the surface of heat exchangers having a high temperature and owing to the temperature of phase conversion upon condensation of released vapour.
Further, all water available in gypsum turns into vapour and is collected in the upper zone of a respective compartment. The dried gypsum descends into the next compartment under the action of this vapour.
Heating devices, pipelines and control and measurement equipment are required for heating oil so that the apparatus for semicontinuous processing of gypsum is rather complicated as a whole and is explosion- and fire-hazardous. In addition, the apparatus for calcination of gypsum is rotated by a vertical shaft so that a special drive and additional energy consumption are required. The method is not continuous so that it cannot be regarded as suitable from the point of view of the up-to-date manufacturing requirements.
The use of this method and apparatus for preheating concrete mix is very difficult. The concrete mix heating time by this method will be too long since heating primarily occurs in the zones of direct contact between the mix and heat exchange surfaces, the adjacent layers being heated only through heat conductance of the liquid component of concrete mix (water). This will result in a high non-uniformity of temperature through the body of concrete mix. In addition, concrete mix will burn at the surface of heat exchangers. They will be rapidly covered with a crust of dry concrete so as to result in a high thermal resistance. First, this will prolong the heating time and then it will make the heating impossible.
Known in the art is an apparatus for continuous electrical heating of concrete mix (cf. USSR Inventor's Certificate No. 874714, Int.Cl. C 04 B 41/30; publ. in Off. Bull. No. 39, 1981). The apparatus comprises a vessel of open cross-section having loading and discharge pipes at the ends thereof. Electrically insulated plate electrodes are provided in the vessel. Each electrode is electrically connected to a voltage regulator.
The ratio of electrode length to height is 1.5-2:1. The apparatus also has one source of oscillations (a directional vibrator) mounted on the rear wall of the loading pipe.
The directional vibrator imparts to the vessel harmonic oscillations, and concrete mix available in the vessel will move along the electrodes. Alternating current is fed to the electrodes via the voltage regulators to pass through the concrete mix and heat it. This method makes it possible to create uniform electric and temperature field so as to avoid local overheating of the mix, boiling and moisture loss at the ends of the heating zone and underheating of the mix in the middle of this zone. Efficiency is 0.94-0.96, and the average heating temperature is increased to 95.degree.-96.degree. C.
However, free contact with the environment during heating of concrete mix results in heat losses from the mix through evaporation. This lowers efficiency of the apparatus in operation and increases energy consumption for mix heating.
Temperature gradient in the liquid phase of concrete mix, which comprises such components as water, binder and sand, is 12.degree. C. over the cross-section of the apparatus. Only liquid phase has time to heat to 95.degree.-98.degree. C. in the zone directly adjacent to the last electrode. The time for heating concrete mix to an average temperature of 70.degree.-80.degree. C. per 1 m.sup.3 of the mix is the same as that in intermittent-action apparatuses. Therefore, the apparatus cannot ensure an improvement of throughput capacity in placing heated concrete mix. If the throughput capacity is increased, the average temperature of concrete mix heating decreases respectively which, in the end of the day, results in a lower concrete strength.
Known in the art is an apparatus for processing a mix, preferably concrete mix (cf. Information Leaflet No. 20684. Vladimir. Scientific Information Center, 1984). The apparatus comprises a closed vessel having loading and discharge pipes at the ends thereof. Three electrodes provided within the vessel coaxially therewith are secured to hinged non-sealed covers. The electrodes are connected to 380 V three-phase mains. A source of oscillations (vibrator) is installed on the vessel.
Concrete mix is continuously supplied to the loading pipe. Voltage is applied to the electrodes, and the vibrator is switched on. Under the action of oscillations, the concrete mix starts moving along the electrodes. Concrete mix being electrically conductive, it will short-circuit all three electrodes in succession and will be heated as ohmic resistance. Having passed through the heating zone, the mix will be fed into a mold or form through the discharge pipe.
In comparison with the apparatus disclosed in USSR Inventor's Certificate No. 874714, this apparatus ensures a better temperature uniformity of liquid phase of the mix, within 8.degree.-10.degree. C. This is due to the provision of a closed vessel. Evaporation of moisture from the concrete mix being heated occurs under the conditions of a limited free contact with outside air. The electrodes are not covered with concrete during operation as the continuously moving mix will clean the electrodes.
The electric power requirements are three times as low as compared with intermittent-action apparatuses having the same throughput capacity. With the same electric power consumption as in case of cyclical electric heating of the mix, the throughput capacity of the apparatus can be three times as great.
However, the absence of sealing of the inner space of the vessel, especially in the zone of direct heating of the concrete mix during its continuous movement, causes heat losses from the mix through evaporation.
As a result, efficiency of mix heating decreases thus requiring an additional energy consumption to make up for such heat losses. Further increase in concrete mix heating temperature is not possible since the interior of the apparatus permanently communicates with the environment during continuous movement of the mix being heated so that maximum temperature of concrete mix cannot be above 100.degree. C., and it is impossible to intensify concrete mix heating by increasing temperature of liquid phase.
The main object of the invention is to provide a method and apparatus for processing a mix, preferably concrete mix which make it possible to intensify mix temperature increase.
Another, not least important object of the invention is to provide a method and apparatus for processing a mix, preferably concrete mix which allow electrical energy consumption for mix processing to be lowered.
These and other objects are accomplished by the provision of a method for processing a mix, preferably concrete mix, comprising continuously moving the mix within a closed vessel, heating the mix during movement by causing electric current to flow therein and subjecting the mix to the action of oscillations, wherein, according to the invention, a sealed zone is created in the vessel, the mix being heated to 100.degree. C. and above in this zone, vapour, which is formed, penetrating the whole body of the mix to carry out uniform and rapid heating of all components of the mix.
The method according to the invention makes it possible to carry out a rapid, easy and economical heating of concrete mix to 100.degree. C. and above without using any external sources of heat and pressure, with the employment of the most widely available and efficient energy source -- electricity.
Electric energy turns to thermal energy during continuous movement of the mix within the sealed zone and directly within the body thereof. Expanded air and vapour released from the mix being heated build-up gauge pressure in the sealed zone so that the electrical heating might be continued to 100.degree. C. and above. The resultant vapour, owing to its low viscosity and high kinetic energy, rapidly and deeply penetrates all micropores of aggregates of the concrete mix and gets to grains of cement to be condensed therein and give up heat so as to heat them. Better penetration of water to cement grains results in an improved activity of cement hydration. Combining direct heating of the mix with the phase transformation heat which is released upon condensation of the resultant vapour makes it possible to combine advantages of the two methods of concrete mix heating: steaming and electrical heating. This results in a more rapid mix temperature rise so as to improve throughput capacity of continuous mix heating plants and lower electric energy consumption. Vibratory stirring occurring concurrently with the mix heating contributes to uniform distribution of all mix components over the entire volume which will improve structural uniformity of concrete. Therefore this method intensifies mix heating and raises final mix temperature after the heating. At the same time, an increase in the heat capacity of the mix contributes to an accelerated hardening of concrete, hence, to shorter time of erection of structures and installations.
Concrete mix is preferably subjected to deaeration and turbulization after heating to 100.degree. C. and above.
This facility provides for air removal from the heated mix and improves uniformity of the mix temperature before placing it into a mold or form.
Air presence in concrete mix is known to lower strength of concrete members and structures. Reduction of air content in concrete mix contributes to an increase in concrete density and strength and improvement of surface finish of members and structures, which is without pores and cavities.
Turbulization of the mix before placing into a mold or form results in destruction of cement grains swallen as a result of hydration so as to remove therefrom "screening films" and to provide for free access of mixing water to the active surface of cement. This results in a deeper hydration of cement grains and, finally, in a more intense growth of concrete strength. In addition, as a result of turbulization of the mix before placing, high uniformity of liquid phase temperature in the mix is ensured, which is within .+-.2.degree.-3.degree. C. and, since turbulization materially intensifies heat exchange between components of the mix, heating of coarse aggregate of the mix is also accelerated. This is favourable for subsequent hardening of the placed concrete because the temperature uniformity of the mix ensures uniform distribution of strength in the concrete of a member or structure.
The problem on which the invention is based is also solved by providing an apparatus for carrying out the method for processing a mix, preferably concrete mix, comprising a closed vessel having loading and discharge pipes at the ends thereof, at least one electrode within the vessel, and at least one source of oscillations, wherein, according to the invention, a gate is provided in the vessel upstream the discharge pipe for controlling the cross-sectional area of the flow path of the vessel.
The provision of the gate makes it possible to completely fill the vessel with the mix and to create a sealed zone therein. As a result, the mix can be heated to 100.degree. C. and above owing to the transformation of electrical energy to heat energy and owing to the phase transformation heat upon condensation of vapour released from the mix being heated. Heating the mix to 100.degree. C. and above makes it possible to intensify mix heating and reduce electric energy consumption.
A shutter is preferably provided at the outlet of the discharge pipe to define with the gate a chamber for deaeration and turbulization of the mix, the shutter being mounted for controlling the cross-sectional area of the flow path of the chamber.
One of the functions of the chamber for deaeration and turbulization of the mix is to remove air available in the heated mix, the presence of the air being detrimental to concrete strength.
Another function of the chamber is to turbulize the heated mix before placing. This improves temperature uniformity of the mix, hence, ensures uniform distribution of strength in the concrete of a member or structure during hardening.
Simplicity of the chamber design and combining it in one and the same apparatus with the vessel in which continuous treatment of the mix occurs make it possible to perform the whole complex of production operations at one point (adjacent to the point of concrete mix placing) simultaneously. This ensures high efficiency of the plant, makes it possible to intensify the mix temperature rise and lowers electric energy consumption.
The gate preferably comprises two series-mounted plates of which one plate, which is closer to the vessel, is mounted for controlling the cross-sectional area of the flow path of the vessel and the other plate is rigidly secured and has a curvilinear central portion having a convexity facing towards the loading pipe and ports located in the lower part of the plate substantially adjacent to the side walls of the vessel, the shutter being made in the form of two series-mounted plates of which one plate, which is closer to the gate, is mounted for controlling the cross-sectional area of the flow path of the chamber and the other plate is rigidly secured and has a concavity facing towards the loading pipe and a port located in the lower part of the plate, in the middle thereof, the area of this port being smaller than, or equal to the total area of the gate ports.
The provision of the rigidly secured plate of the gate which is so constructed contributes to splitting of the mix flow in two and also to the cleaning of the surface of this portion of the gate by the moving mix. The controllable plate is necessary for shutting-off the cross-sectional area of the flow path of the vessel for its complete filling with the mix and for creating the sealed zone at the beginning of heating. Under steady mix heating conditions the controllable plate is normally set above the ports and retained in this position. It is also possible to vary the area of the ports during mix heating by means of the controllable plate, hence to vary throughput capacity of plant and temperature of mix heating. After the two streams of the mix have passed through the gate, they move to the chamber wherein the mix is deaerated and turbulized.
The provision of the rigidly secured plate of the shutter which is so constructed makes it possible to carry out turbulization of the mix before placing it into a mold or form since the two streams of the heated mix formed by the gate will pass through the chamber to hit against a barrier in their path, i.e. against the rigidly secured plate of the shutter so that they will abruptly change the direction of movement to intersect each other thus turbulizing the mix as a whole. The heated mix will then escape through one common port located in the lower part of the rigidly secured plate, in the middle thereof. As a result, high uniformity of temperature of liquid phase of the mix before placing into a mold or form is ensured, within .+-.2.degree.-3.degree. C.
The area of the port of the rigidly secured plate of the shutter should be smaller than, or equal to the total area of the ports of the rigidly secured plate of the gate. This facility provides for free escape of the heated mix from the chamber without the risk of a dead zone being formed therein. Failure to comply with this requirement would result in an interruption of mix placing into a mold or form, and a plug may form in the vessel thus causing an interruption of the mix heating process and requiring cleaning of the interior of the vessel to remove hardened mix from the chamber.
Curvilinear portions having a concavity facing towards the loading pipe are preferably provided in either side of the central curvilinear portion of the rigidly secured plate of the gate.
This facility provides for a better passage of the heated mix through the ports of the gate since formation of dead zones of the mix adjacent to the gate is completely eliminated. Therefore, the gate ports will not be clogged with concrete.
The gate is preferably made in the form of two series-mounted plates of which one plate, which is closer to the vessel, is mounted for controlling the cross-sectional area of the flow path of the vessel and the other plate is rigidly secured and has a concavity facing towards the vessel and a port located in the lower part of the plate, in the middle thereof, the shutter being made in the form of two series-mounted plates of which one plate, which is closer to the gate, is mounted for controlling the cross-sectional area of the flow path of the chamber and the other plate is rigidly secured and has a curvilinear central portion having a convexity facing towards the chamber and ports located in the lower part of the plate substantially adjacent to the side walls of the chamber, the area of the ports being smaller than, or equal to the area of the gate port.
The provision of the rigidly secured plate which has a concavity facing towards the vessel and the port located in the lower part of the plate, in the middle thereof, makes it possible to concentrate the flow of heated mix at the central portion of the gate thus ruling out formation of dead zones upstream the gate and contributing to cleaning of the surfaces of the gate with the moving mix. The controlled plate is necessary for shutting-off the cross-section of the flow path of the vessel so as to completely fill it with mix and create the sealed zone at the beginning of heating. Under steady heating conditions the controlled plate is normally set above the ports and retained in this position. The area of the gate ports can also be varied during mix heating by means of the controlled plate so as to control throughput capacity of the plant and mix heating temperature. After the passage through the gate the mix stream is fed to the chamber for deaeration of the mix and its turbulization.
The provision of the rigidly secured plate of the shutter which is so constructed makes it possible to carry out effective turbulization of mix before placing it into a mold or form since the resultant stream of heated mix that passes through the chamber would hit against a barrier in the form of the rigidly secured plate of the shutter in its path to be split into two independent streams. At the same time, this portion of the shutter is cleaned by the moving mix. The resultant two streams will hit against the chamber walls and will then pass through the ports in the lower part of the plate which are located substantially adjacent to the side walls of the chamber. Thus each stream and the mix as a whole are turbulized before placing into a mold or form. As a result, high uniformity of temperature in the liquid phase of the mix within .+-.2.degree.-3.degree. C. is ensured before placing into a mold or form. The effect of the ports of the shutter having the area which is smaller than, or equal to the area of the gate port is similar to that described for the abovementioned embodiment.
Curvilinear portions having a concavity facing towards the loading pipe are preferably provided on either side of the central curvilinear portion of the rigidly secured plate of the shutter.
This facility ensures a better passage of the mix through the ports of the shutter since the possibility of formation of dead zones of the mix adjacent to the shutter is completely eliminated. Hence, there will be no clogging of shutter ports with concrete.
The gate and shutter in the apparatus for processing a mix, preferably concrete mix preferably have self-closing valves mounted on the rigidly secured plates of the gate and shutter, respectively.
The provision of the self-closing valves ensures their reliable and permanent intimate contact with the surface of the moving mix under any fluctuations of the mix level in the vessel and chamber so as to protect the interior space of the vessel and chamber against penetration of outside air, hence against heat losses from the heated mix. In addition, the use of the self-closing valve on the shutter ensures removal of only air from the mix when it passes along the chamber. There will be no moisture evaporation nor heat loss from the chamber since temperature in the chamber and in the mix will be one and the same under steady conditions of operation of the apparatus.
Therefore, the use of the method and apparatus for processing a mix, preferably concrete mix according to the invention makes it possible to intensify mix temperature rise and lower electric energy consumption.